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Saturday, September 27, 2014

I previously posted a picture of an SNR to MCS data rate mapping table that I have compiled based on various sources of credible research. Keith Parsons has kindly put this information into a printable format for reference. You can download them below.

It should be noted that individual devices perform differently. These tables are simply generic estimates that are a good approximation for many Wi-Fi devices. In other words, it's not perfect.

This table maps client SNR values to MCS indexes for the purpose of determining the data rates that clients can achieve based on the signal quality of their connection to the AP.

SNR is also related to RSSI. Two RSSI values are of importance: the Minimum Receiver Sensitivity and the Expected Receiver Sensitivity. The 802.11 minimum receiver sensitivity tables often referenced in research and testing material are the required minimum RSSI values that a radio should be able to decode a given modulation type and encoding rate (MCS index) with a packet error rate (PER) less than 10%. Most 802.11 radios provide better receiver sensitivity than the minimum requirement. Therefore, the "Expected Receiver Sensitivity" reflects the typical receive sensitivity of clients with the ability to achieve any given MCS index at a lower RSSI than the minimum receiver sensitivity required to pass testing. For example, the minimum receiver sensitivity for an 802.11ac 20 MHz PPDU at MCS 9 is -57dBm, but most 802.11ac radios can decode this PPDU at a lower RSSI such as -62dBm.

It should also be noted that a receiver's ability to perform Maximal Ratio Combining (MRC) across multiple receive antenna chains is not reflected in this SNR chart. MRC can allow a device to receive the incoming signal at a lower energy level at each of the individual antenna inputs to the RF front-end radio circuitry which are then combined using digital signal processing (DSP) to provide additive gain. This effectively increases the SNR the client experiences. MRC is based on each client device's receive antenna chain specifications and the number of spatial streams being used for the link between the client and AP, with extra receive radio chains being used for MRC. After MRC gain is added, you can use this table to lookup the MCS rate the client may be able to achieve given it's final resulting SNR . Also be aware that many manufacturer receive sensitivity specifications will list RSSI and SNR values 3-6 dB lower than what is specified here because they list the signal level at the antenna input prior to DSP and MRC gain.

Some of the references used to help compile this table (not an exhaustive list):